Two-phase polaron model of conduction in magnetite-like solids

A model and an approximate theory is presented for conduction in solids which have a charge-ordered low-temperature phase and show a first-order phase transition of 'semiconductor-semimetal' type at a characteristic temperature T nu (e.g. Fe 3 O 4 ). The structure and the conduction mechanisms of the low-temperature phase are analysed. It is shown that the phase transition is of first order; an approximate treatment of the transition is proposed for obtaining a simple relation between T nu and the correlation parameter. The structure of the high-temperature phase is discussed and the main features of the conduction are considered. The behaviour and mechanisms of the steady and frequency-dependent conductivity, generally speaking, are essentially different at temperatures higher and lower than T nu .